Do I HAVE to register my trademark?

It has come to my attention that seemingly a lot of trademark lawyers have been telling their clients that they "have" to register their trademark to protect it.  That drives me nuts. 

I can't stand when people tell half truths.  All the fine print garbage makes me crazy.  I'm a full disclosure kind of girl. 

I'm not going to say that federal registration of your trademark is not a good idea.  Of course, federal registration is certainly helpful in protecting your mark.

But is it necessary?

The answer may surprise you, but it's not necessary to register your trademark with the United States Patent and Trademark Office (USPTO) to protect your mark against infringement.

Under U.S. law, you gain trademark protection once you have used the mark "in commerce". This is often referred to as a common law trademark.

You can use the "TM" indication (or "SM" for a service mark) after your mark at any time after beginning to use it in commerce (though you can only use the ® mark once you have a registered mark with the USPTO).

So, why do people register their marks with the USPTO?